ES10 - EARTH

Lecture 15 - Igneous and metamorphic rocks

Lisa Tauxe

In the last lecture, we zoomed in on the weathering/sedimentation part of the rock cycle.
Today, we will consider what happens when stuff goes down the tubes, gets metamorphosed and
ultimately gets remelted and comes up as igneous rocks once again.

Metamorphism - what is it?

Metamorphism means to change form. In the geologist's sense it refers
to changes in rocks (protoliths) in the solid state (i.e. not by
melting the rock wholesale). One can view metamorphism as similar to cooking.
Ingredients are mixed and placed at a different temperature (and/or pressure)
and changes occur. What was a batch of goo, turns into a lovely cake!

Change occurs in order to maintain equilibrium conditions with new states
of heat, pressure, or fluids. Thus, major changes in any of these three
environmental variables can result in metamorphism. Because of the great
conveyor belt of plate tectonics, rocks can encounter a variety of environmental
conditions riding around in pressure, temperature, fluid space. As they
go, they may metamorphose and may have a tale to tell of where they've
been.

The fingerprints of metamorphism are growth of new minerals stable at
the new PTF conditions and changes in texture reflecting the state of stress.

Temperature

There are several sources of the thermal energy that drives metamorphism.
Obviously, when and igneous body (some 1200 degrees C) intrudes into unsuspecting
host rock, the contact zone heats up considerably. This causes a baking
of the neighboring rock called contact metamorphism.

The temperature in the Earth goes up with depth. Near the surface in
ordinary crust, the temperature rises some 30 degrees per kilometer. This
is partly due to radioactive decay within the crust, but also comes from
the fact that the Earth is still very hot from its initial formation (recall
that the core is largely molten iron!). So if rocks are taken from the
surface "down the tubes" either by burial or on a lithospheric
nose dive, they will heat up and metamorphose.

Pressure

Pressure changes with depth in the Earth as a result of the increased
weight of the overlying rock. Increased pressure drives minerals to form
more compact phases, driving for example coal to change into diamonds,
and clay to rubies. Pressure also changes the state of stress. Crystals
will grow or deform by cracking or flowing in response to the change in
stress and show either preferential alignment, or evidence of squashing
that reflects in some way the stress regime of the new environment. Deformation
near fault zones, for example, results in cracking and grinding of rocks
and is referred to as cataclastic metamorphism, while deformation
deep in the crust occurs more plastically over a wide area producing regional
metamorphism.

Fluids

Changes in the chemistry of the fluids in the pore spaces of rocks also
induces change. In its lowest temperature/pressure form, this change is
called diagenesis, including weathering discussed in the
last lecture. Diagenesis is not usually considered part of metamorphism
although the distinction is a pretty subtle one if you ask me.

One common cause of changes in the fluid chemistry is the proximity
of something hot - take volcanic activity for example. The heat induces
convection currents in the surrounding fluids. The heated water reacts
locally with the hot rock and carries a load of dissolved matter (rich
in metals and sulfer) from the region of high heat into a region of cooler
rock (or water). Here, the fluids tend to "dump their load".
This form of metamorphism is known as hydrothermal alteration and
is the way most metallic ore bodies formed. When the super-charged fluids
come out of brand-new ocean crust and hit 2C water, they drop their load
and form what is known as a black smoker. Lots of animals (if you
can call them that) live off the stuff. Check these out:

Metamorphic textures

Increasing pressure causes a reduction in volume of the rock by first
compaction and then recrystallization of minerals to denser
forms. Compaction produces a more closely packed arrangement of grains.
This is exmplified by the transformation of clay to slate. Recrystallization
involves growth of new minerals. The bulk chemistry need not change (unlessfluids
are involved).

If the pressure is higher in one direction than the others, minerals
will tend to align themselves such that the fabric formsperpendicular to
this axis. A planar fabric is known as a foliation while alignment of
crystals causes a lineation.

foliation

Micas (which are platey) will grow with their plates along the foliation.
Increasing metamorphism resultsin different types of foliation.

In its journey around the tectonic merri-go-round, clay will turn
first to shale during diagenesis, then procede to slate. Growth of micas
then will contribute a peach fuzz sheen characteristic of phyllite.
As metamorphism
becomes more intense, schist forms. Schists have large mica flakes
in them.

If the rock easily splits along smooth parallel surfaces, it has
what is known as fracture cleavage.

Non-foliated rocks

Not all rocks are foliated. For example, contact metamorphism does
not generally produce foliated rocks. Also, parent rocks (protoliths) that
tend to grow minerals that are not platey or elongated, will produce metamorphic
rocks that have no foliation or lineation. These include quartz, and calcite.
So sandstones make massive quartzites and limestones make marble, neither
of which are strongly foliated. Here is an example of what
marble
looks like.

Porphyroblast texture

Porphyroblasts are larger crystals in a finer-grained matrix. The
metamorphic rock will bear the name of the dominant porphyroblasts, e.g.
garnite schist.

Mineral changes in metamorphic rocks

Changing conditions results in phase transformation from one mineral
to another. Minerals coalesce or change crystal structure. The particular
minerals that form are characteristic of thepressure/temperature conditions.
Particularly useful for determining PT conditions are the following
metamorphic
index minerals

High metamorphic grade - 800 degrees C (verging on melting), such
as gneiss and migmatite have the high temperature high pressure phase sillimanite.

Staurolite, kyanite and sillimanite all have the same composition
but are stable at different PT conditions (like graphite and diamond).
Therefore the presence of one particular form documents the PT conditions.
A more accurate idea of PT conditions can be gotten by considering a
whole suite
of minerals. Determining the PT history of a sequence of rocks describes
the journey of that particular crustal package up and down the tectonic
elevator.

Role of initial composition

Obviously, the composition of the protolith plays a strong role in
which minerals will grow. Thus basalts, granites and carbonate rocks each
develope into the different metamorphic mineral assemblages leading ultimately
to amphibolite, gneiss or marble respectively.

Plate tectonic settings of metamorphism

Hydrothermal metamorphism occurs
near mid-ocean ridges driven by the heat of the volcanic activity there.
Intrusion of igneous rocks drives contact metamorphism anywhere it occurs.
Both of these sorts are metamorphism with high temperatures and low pressures.

Cataclasis grades into totally pulverized minerals that are streaked
out in bands characteristic of mylonites.

blue-schists. When
found on the surface, they are a strong indication that there once was
a subduction zone in the neighborhood.

Finally, burial of sediments in a sedimentary basin takes the rocks
down the PT road characteristic of the crust, the so-called geotherm
.
They respond to this by developing the characteristic mineral phases of
burial metamorphism. The minerals are a guide to just how deep and hot
sediments got.

Where (and why) do igneous rocks form?

Rocks melt when the temperatures, pressures, and/or water content are
sufficient to bring the rock to the melting tempertare (duh). This
most occurs 1) when rocks have been brought up from deeper in the
mantle for example at a RIDGE, 2) rocks with water in them are
brought from the surface down into the mantle, for example at a
SUBDUCTION ZONE, or there is some "HOT SPOT" arising from deep in the
mantle unrelated to a plate boundary. About 90% of all melting occurs at
plate boundaries (spreading or subducting). The rest is called
"intra-plate" and is typified by Hawaii. For a summary of
where melts form, see Figure 4.7 in your book.

When a partial melt forms, it rises and collects in a magma chamber (see
Figure 3.2 in your book). In the magma chamber, the melt continues to
crystallize thus changing its chemistry. This is a process known as
magmatic differentiation. As magmas cool, different minerals
will crystallize out of the melt. By studying the crystallization of
melts in the laboratory, this process is fairly well understood.
If these minerals settle out of the melt to the floor of the
magma chamber, (see Figure 4.11 in your book), the chemistry of the
remaining melt changes from a more mafic to a more felsic melt; thus, if
fractional crystallization is taken to the extreme granite can be gotten
from what was originally a basaltic melt.

The magma chamber may erupt from time to time. If the melt doesn't make
it to the surface, it forms an intrusive rock. (see Figure 4.16
in book). Intrusive bodies can be big balloon shapes (plutons),
sub-horizontal slabs (sills) or sub-vertical walls (dikes).

If it does make it, it
becomes an extrusive rock. Extrusives can flow out over the
ground (lava flows) or be blasted into the air to form ash
falls and pyroclastics.

Classification of igneous rocks

Igneous Rocks have a two-dimensional
classification scheme based on
chemistry, grain size and texture.

chemistry:

The key to chemical classification in igneous rocks is the amount of Silica
(SiO2) in the magma. (Of course people who study this make a much bigger
deal out of it!
If magmas don't have much silica, their minerals are dominated by
magnesium and iron (Fe) - hence the term MAFIC (MA- from the magnesium
and FIC from the Fe), or even ULTRAMAFIC for the really silica poor
varieties. Silica rich magmas have a mineral named feldspar
in them (see book) and are called FELSIC as a result. You will also
see the words "acidic" and "basic" used for felsic and mafic
respectively and you should be aware that this has nothing to do with
pH! One can often tell about how much silica is in a rock just by its
color. The more silica, the lighter the color.

grain size:

The main control of grain size is how fast the rock cooled from the
molten state. Slow cooling allows bigger crystals to form, and fast
cooling makes smaller crystals and even glass (no crystals).
So the
second dimension of igneous rock classification is whether the rock was
formed by cooling on the surface as an extrusive rock. or in
the crust as an intusive rock. Magma can
either be erupted (extruded) as ash to make pyroclastic
rock or as lava to make volcanic rocks.

IGNEOUS COMPOSITIONAL NAMES AND MAGMA TYPES

SiO2 (wt. %)

<45

45 -52

52 - 57

57 - 63

63 - 68

>68

Compositional or Chemical Equivalent

Ultrabasic

basic

basic to intermediate

intermediate

intermediate to acidic or silicic

acidic or silicic

Magma Type

ultramafic

mafic

mafic to intermediate

intermediate

intermediate to felsic

felsic

Extrusive Rock Name

komatiite

basalt

basaltic andesite

andesite

dacite

rhyolite

Intrusive Rock Name

peridotite

gabbro

diorite

diorite or quartz diorite

granodiorite

granite

Liquidus Temperature

Mafic Mineral Content

Water Content

Mg/Fe

Ca/Na or Ca/K

Texture

Igneous textures are classified by the presence or absence of
crystals, the size of the crystals, and the size and density of
vesicles (holes).
Check out this page for a nice summary of igneous
textures.

Extrusive rocks

Pyroclastic rocks:

Pyroclastic rocks are classified by grain size from BOMBS (>64mm) to
ash (<2mm). Lapilli are pea-like grains often in a finer matrix.

Here is a nice picture I found to illustrate the classification scheme
of pyroclastics:

Volcanic Rocks:

Volcanic rocks are mainly classified by the amount of silica. There are
four main categories with increasing silica: basalt, andesite, dacite
and rhyolite.

Intrusive Rocks

Intrusive rocks cool slower and have coarser grain sizes than their
extrusive counterparts. The big four of intrusive rocks are with
increasing silica: gabbro, diorite, granodiorite, and granite.

A few other points

Viscosity:
Increasing silica not only makes magmas lighter (in color), but makes
them more viscous (stiffer), so the beautiful movies of flowing glowing
lava flows from Hawaii are of basaltic lavas with little silica.

Melting temperature:
Increasing silica lowers the melting temperature, so that
granites melt at about half the temperature that basalts do.

Water: Adding water also lowers melting temperature and decreases
viscosity. Also, water and other gases make bubbles in the magma, contributing
to the explosive power of some eruptions and also leaving holes in the
rocks (vesicles).

Melting: While we're on the subject of melting, it is very important
to note that each mineral has a different melting temperature, so rocks
do not melt all at once, but bit by bit. Thus most magmas have floating
bits of crystals in them (phenocrysts) and are not 100% melted.